Why would I want to take prisoners?

There is a tip on one of the loading screens explaining that I can take certain people I meet prisoner. There is also an upgrade for my Stronghold that is required to do that.

What do I gain by restoring my dungeon and then imprisoning people? I'm trying to decide whether to upgrade that part, and when to upgrade. I'd appreciate a mostly spoiler-free version without story specifics, I'm mostly interested in the general advantages of building the dungeon and imprisoning people.

Solution 1:

"Visitors

The most common event at your stronghold is having a visitor arrive. There are all kinds of visitors, but they all share one thing. They can adjust your prestige and/or security just by being at your stronghold. Some visitors give good bonuses and some of these visitors can even be employed as hirelings and will stay on as long as you pay them. Others are not so great, and you can offer them one of your companions to act as an escort to their next location, or pay them to leave. Some visitors will offer rare items for sale, and some might even offer a very rare item in exchange for one of the prisoners in your dungeon. If Visitors die during their stay, the stronghold suffers a Prestige penalty for a while."

http://pillarsofeternity.gamepedia.com/Stronghold?cookieSetup=true